Brainless bristlebots show swarm behaviour

Think of swarms, and anything from bees to fish to cooperating robots may come to mind. But vibrating toothbrushes?

They're called bristlebots - super-simple robots that skitter wildly across the ground. You can make one at home with a severed toothbrush head, bit of tape, a motor from a pager (because you sure don't need that pager anymore) and a watch battery. Bristlebots have no CPU and no sensors. All they do is vibrate and shift a tiny bit - about 10 micrometres - with each vibration. Depending on how their bristles are oriented, they will either zip into a swerving forward march or spiral into a sideways spin.

A bristlebot's motions look random. But Luca Giomi, an applied mathematician at Harvard University, and his colleagues wanted to see whether groups of bristlebots actually form coordinated swarms like living creatures and smarter robots do. So they assembled a bunch of the little fellers, each with the same elliptical design, put them in an arena, and watched them go.

As long as the robots take up at least 8 per cent of the arena space, the bristlebots gather into swarms, sliding along the edge of the arena en masse (see video). The behaviour demonstrates how apparently intelligent behaviour like swarming doesn't require thought at all, says Giomi.

"The bristlebots are just bumping into each other and bumping on the edge of the area," he says. "They are able to sense their neighbourhood and environment, and because of this contact interaction, they are able to pass this information to one another. This is enough to establish some coordination."

For other examples of emergant behaviour, what percentage addition is needed to make fibres in paper making turn their sludge into something near immobile, or grains in a silo to lock together or cars in a city to cause jams etc?